This invention relates to lead-free frangible bullets having improved frangibility and novel processes for producing these bullets.
The advantages and desirability of lead-free frangible bullets is described in co-pending U.S. patent application Ser. No. 08/678,776, filed Jul. 11, 1996, the entire disclosure of which is incorporated by reference herein.
Traditionally bullets for small arms ammunition have been manufactured from lead and lead alloys. The major advantages of lead as a bullet material are its relatively low cost, high density and high ductility. The high density of lead has been particularly important to bullet design because the energy generated by the weight of a bullet is critical to the proper functioning of modern semi-automatic and automatic weapons, the in-flight stability of the round, and the terminal effects of the bullet.
The highly toxic nature of lead, however, and its propensity to fume and generate airborne particulate, place the shooter at an extreme health risk. The more a practice firing range is used, the more lead residue builds up, and the greater the resulting lead fume and lead dust pollution (particularly for indoor ranges). Moreover, the lead bullet residue left in the earthen berm of outdoor ranges can leach into the soil and contaminate water tables. In order for indoor ranges to operate safely, extensive and expensive air filtration systems are required. Both indoor and outdoor ranges require constant de-leading. These clean-up operations are time consuming, costly and repetitive. Accordingly, there is a great need for lead-free bullets.
Additionally, personnel at range operations are concerned with the ricochet potential and the likelihood of causing xe2x80x9cback-splatterxe2x80x9d of the training ammunition. Back-splatter is a descriptive term for the bullet debris that bounces back in the direction of the shooter after a bullet impacts on a hard surface, such as steel targets or backstops. Both ricochets and back splatter present a significant hazard to individuals, equipment and structures in and around live firing ranges. A ricochet can be caused by a glancing impact by a bullet on almost any medium. When a bullet strikes a hard surface at or near right angles, the bullet will either break apart or deform. There is still energy in the bullet mass, however, and that mass and its energy must go somewhere. Since the target material or backstop is impenetrable, the mass bounces back in the direction of the shooter.
It is believed that a key way to minimizing the risk of both ricochet and back-splatter is to maximize the frangibility of the bullet. By designing the bullet to fracture into small pieces, one reduces the mass of each fragment, and in turn the overall destructive energy remaining in the fragments.
Several prior art patents disclose materials and methods for making non-toxic or frangible bullets or projectiles. For example, U.S. Pat. No. 5,442,989 to Anderson discloses projectiles wherein the casing is frangible and made out of molded stainless steel powder or a stainless steel plus pure iron powder mix with up to 2% by weight of graphite. The casing encloses a penetrator rod made of a hard material such as tungsten or tungsten carbide. This projectile is mainly for 20-35 mm cannons to engage targets such as armored vehicles, trucks, buildings, ships, etc. Upon impact against the target, the casing produces fragments which are thrown in all directions with great energy while the penetrator rod pierces the target.
U.S. Pat. No. 4,165,692 to Dufort discloses a projectile with a brittle sintered metal casing having a hollow interior chamber defined by a tapering helix with sharp edge stress risers which provide fault lines and cause the projectile to break up into fragments upon impact against a hard surface. The casing is made of pressed iron powder which is then sintered. This projectile is also designed for large caliber rounds such as 20 mm cannon shots.
U.S. Pat. No. 5,399,187 to Mravic et. al discloses a lead-free bullet which comprises sintered composite having one or more high density powders selected from tungsten carbide, ferrotungsten, etc., and a lower density constituent selected from tin, zinc, iron, copper or a plastic matrix material. These composite powders are pressed and sintered. The high density constituent allows bullet densities approaching 9 g/cm3.
U.S. Pat. No. 5,078,054 to Sankaranarayanan et. al discloses a frangible projectile comprising a body formed from iron powder with 2 to 5% by weight of graphite or iron with 3 to 7% by weight of A2O3. The powders are compacted by cold pressing in a die or isostatic pressing, and then sintered.
U.S. Pat. No. 5,237,930 to Belanger et. al discloses a frangible practice ammunition comprising compacted mixture of fine copper powder and a thermoplastic resin selected from nylon 11 and nylon 12. The copper content is up to about 93% by weight. The bullets are made by injection molding and are limited to densities of about 5.7 g/cm3. A typical 9 mm bullet only weighs about 85 grains.
An objective of this invention is to provide a range of lead-free frangible bullets, optimized for frangibility, which will eliminate the lead fumes and dust hazard to the shooter while also minimizing the ricochet and back-splatter hazards. A further objective is to provide a low cost material and process for making such a bullet. Yet another objective is to provide a bullet with a weight and density as high and as close to the conventional lead bullet as possible so that the recoil and the firing characteristics closely resemble those of conventional lead bullet. Yet another objective is to reduce the risk of lead residues leaching into the soil and water table in and around shooting ranges.
Another objective of this invention includes providing processes of manufacturing frangible bullets that enable the production of compacted bullets with extremely small dimensional changes occurring between the pressed compact and the final product, thus making it much easier to predict finished product bullet dimensions. An additional objective of this invention is the avoidance of high sintering temperatures and times and the associated high energy costs.
The present invention provides frangible bullets or projectiles and materials and processes for the manufacture of such bullets and projectiles. More particularly, the bullets of the present invention comprise a compact of powder particles of a first (matrix) metal bonded together with a binder of a second (binder) metal.
In a preferred embodiment of the invention, the bullets comprise a compact of matrix metal powder particles having a higher melting point bonded together by a binder metal having a substantially lower melting point. More particularly, the matrix metal powder particles are xe2x80x9cwetxe2x80x9d by the binder metal thereby binding them together.
The bullets of this preferred embodiment may be manufactured by a novel process comprising first admixing combinations of two or more lead-free metal powders wherein at least one of the lead-free metal powders (binder) has a melting point substantially lower than that of the other metal powder (matrix) present. A pressed compact of the admixture of metal powders is formed into the shape of a bullet or projectile as desired, and the pressed compact then is heated under conditions effective to reach the melting point of the lower melting point binder metal to place the lower melting point binder metal into a molten state, or at least a partial molten state, thereby effectively xe2x80x9cwettingxe2x80x9d the surface of the higher melting point matrix metal powder. The process is effective to bond the compacted powders together with minimal alloying taking place.
In another preferred embodiment of the invention there is provided a frangible bullet comprising a compacted mixture of particles of a first (matrix) metal powder, and particles of a second (binder) metal alloy powder, comprised of at least the matrix metal and an alloying metal, wherein the particles are bonded together by a portion of the alloying metal diffusing into the first metal powder particles.
The bullet of this second preferred embodiment may be manufactured by forming a pressed compact of the particles of the two metal powders in the shape of the bullet desired. The pressed compact is then heated under conditions to diffuse at least a portion of the alloying metal from the alloy and into the particles of the matrix metal powder without substantially melting any of the powder particles. The diffused metal binds the powder particles together.
Wherever particles of a pure metal powder and an alloy metal powder are in contact, atoms of the alloying metal attempt to diffuse into the pure metal. This is due to the natural tendency of the alloying atoms to equally distribute themselves among all atoms of the pure metal. When this phenomenon occurs at temperatures where no melting of any type occurs, it is known as solid-state diffusion. It creates bonds between the individual metal particles and the collective effect of these bonds holds the compacted process together. The process is not limited to mixtures of a pure metal powder and an alloy powder as bonds can be similarly created between particles of two alloy powders.
An advantage of this invention is that the above described xe2x80x9cwettingxe2x80x9d bonding process and xe2x80x9cdiffusionxe2x80x9d bonding process of manufacturing versus conventional sintering enables the production of compacted bullets with extremely small dimensional changes occurring between the pressed compact and the final product. Thus, it is much easier to predict finished product bullet dimensions. An additional advantage of the process of this invention is the avoidance of high sintering temperatures and times and associated high energy costs.
This invention is more fully explained below in the Detailed Description of preferred embodiments.